PROJECT SUMMARY/ABSTRACT Imaging is an essential tool to guide stroke patient management. Recent DAWN and DEFUSE-3 trials have provided the definitive evidence that imaging is critical to guide late endovascular therapy (EVT), underscoring the need to transition from onset time-based to tissue-based treatment. However, it has been estimated that the majority of stroke patients (~70%) presenting 6-24 hrs with NIH Stroke Scale (NIHSS) over 6 are not DAWN and/or DEFUSE-3 eligible (Desai et al. J Neurointerv Surg. 2018; 10:1033-37). The 2018 American Heart Association Stroke Early Management Guidelines pointed out that it is ?? reasonable to incorporate collateral flow status into clinical decision making in some candidates to determine eligibility for mechanical thrombectomy.? Tissue metabolic MRI is a surrogate biomarker of the collateral flow status that is promising to address this clinical need. We will develop fast pH and diffusion kurtosis imaging (DKI) and evaluate their value for guiding reperfusion therapy in experimental stroke models. During the previous funding cycle, we made steady progress in developing new stroke MRI techniques. We developed pH-specific imaging based on magnetization transfer and relaxation-normalized amide proton transfer (MR-APT) MRI (Guo et al. Neuroimage 2016;242-9). Using filament middle cerebral artery occlusion rats, we refined the perfusion/diffusion-weighted imaging (PWI/DWI) lesion mismatch for the metabolic penumbra and benign oligemia (Wang et al. Neuroimage 2019;610-7). Our acute stroke patient study showed that the volumes of acute DKI lesions have a higher correlation coefficient than those on the apparent diffusion coefficient (ADC) maps with 1-month follow-up T2 images (Yin et al. Radiology 2018:651-7). This echoes the recent study from NINDS investigators who reported: ?ADC evolution in patients with early, complete revascularization, now more commonly seen with EVT, is strikingly different from our historical understanding.? (Hsia et al., Stroke. 2019;50:2086-92) We hypothesize that new fast MRI techniques refined tissue-clock helps delineate ischemic tissue, predict tissue outcomes in permanent stroke, and guide reperfusion treatment. It is urgent to test the refined tissue-clock, in the presence of common stroke comorbidities and in the experimental setting, before initiating future prospective clinical trials. Briefly, we will develop robust fast pH and DKI stroke imaging (Aim 1), establish the spatiotemporal evolution of the refined MRI in MCAO rats (Aim 2), and test tissue-clock-guided reperfusion in rats of common stroke comorbidities of age and diabetes (Aim 3). The success of the project will lay the groundwork for future clinical trials to test advanced imaging-guided stroke treatment in the clinic.